flat grassshader, like in Dauntless, Fortnite, the Witness, Zelda:botw
Since the suggetions are not working (still getting darker colors of some grassbillboards) :( ,
I managed to build a workaround.

The downside is:


it doesnt work for moving light/objects, because the shadows are baked. ( or is there a way to bake a texture for every single Frame).
it also inst an accurate shadow, because the color stays the same on the Z axis. (So it works best if the grass inst tall)
for large scaled scenes you need huge textures to bake on (10k*10k pixels and up).


How its done:

Bake shadows on a white plane, were your grass is emitted from.
Take the baked texture as an factor between a bright color (light) and a dark color (shadow).
Instead of using the hair particle system, use the emitter system, because it let you project the texture of the emitterplane onto the particles.

But its easier to explain the idea and technik behind this workflow with a picture:



So this question is half solved, thanks so far :D.
 I am also happy to see further ideas to solve it completely.